President Donald Trump is planning to offer Canada and Mexico a temporary exemption from new tariffs on steel and aluminum imports, reversing his original insistence that the measures apply to U.S. allies as well as nations like China, administration officials said Wednesday.

One version of the plan, which was still being finalized ahead of an expected announcement on Thursday, would give Canada and Mexico a 30-day exemption from the tariffs, the officials said. The exemptions could be extended based on progress in renegotiating the North American Free Trade Agreement.

The move comes as the White House signaled a new flexibility after a six-day drama that has roiled relations with the country's closest allies, triggered the resignation of National Economic Council chief Gary Cohn and spooked investors. Republicans in Congress have been urging the president to narrow his proposed global tariffs to avoid boomeranging on U.S. businesses and consumers.

Peter Navarro, the director of the White House's Trade and Manufacturing Policy office, said Wednesday night on Fox Business that the president would meet Thursday at 3:30 p.m. with steel union workers and "sign the proclamations. And within about 15 to 30 days, the tariffs go into effect. The proclamation will have a clause that does not impose these tariffs immediately on Canada and Mexico." Other officials said the timing of the announcement and details of the plan remained fluid and subject to change.

The White House shift came after Defense Secretary Jim Mattis and Secretary of State Rex Tillerson made a last-minute appeal for flexibility, saying that overly broad tariffs would damage key security ties with U.S. allies.

On Capitol Hill, Republican lawmakers accelerated their efforts to pull the president back from a potentially costly trade war that he has insisted would be "easy to win."

Rep. Kevin Brady, R-Texas, the chairman of the House Ways and Means Committee, released a letter signed by 107 House Republicans, urging the president "to tailor" the tariffs to address market distortions caused by Chinese surplus production depressing global metals prices.

"We urge you to reconsider the idea of broad tariffs to avoid unintended negative consequences to the U.S. economy and its workers," the letter read. " ... Because tariffs are taxes that make U.S. businesses less competitive and U.S. consumers poorer, any tariffs that are imposed should be designed to address specific distortions caused by unfair trade practices in a targeted way while minimizing negative consequences on American businesses and consumers."

The party's extraordinary internal split was underscored when the Republican Study Committee, representing more than half of House Republicans, released a statement defending free trade and labeling tariffs "a tax on American consumers and businesses."

Rep. Mark Meadows, R-N.C., the chairman of the Freedom Caucus and one of Trump's most trusted allies in Congress has spoken with the president multiple times over the past week in opposition to the tariffs, said three people briefed on his efforts who were not authorized to speak publicly.

"I've never seen anything like this. 'Chaos' doesn't really do it justice," said Claude Barfield, a resident scholar at the right-leaning American Enterprise Institute.

Government lawyers have struggled in recent days to reconcile Trump's public comments with the legal provisions they have been told to enforce. For example, Trump is trying to use the tariff threats to force Canada and Mexico to offer unrelated concessions in NAFTA. By publicly acknowledging this, he has potentially spoiled the legal standing of the tariffs, a senior administration official said, making it harder for them to design the prohibitions.

Earlier this week, Trump suggested that he would exclude Canada and Mexico from the new levies only if they made concessions in negotiations aimed at reaching a new NAFTA deal. Officials from both countries rejected the demand, with Canadian Prime Minister Justin Trudeau calling the new tariffs "absolutely unacceptable."

Major business groups that are normally allied with the Republican Party joined the anti-tariffs chorus.

"These new tariffs would directly harm American manufacturers, provoke widespread retaliation from our trading partners, and leave virtually untouched the true problem of Chinese steel and aluminum overcapacity," said Tom Donohue, president of the U.S. Chamber of Commerce. "Alienating our strongest global allies amid high-stakes trade negotiations is not the path to long-term American leadership."

The Grocery Manufacturers Association warned that the import taxes would disrupt global supply chains and raise costs for consumers while the Beer Institute chimed in with predictions of 20,000 job cuts by its members.

The American Institute of Architects said the import levies would "drastically increase" the cost of building materials, threatening the viability of the president's infrastructure proposal.

Republican lawmakers also want the president to establish a "robust exclusion process" when he announces the tariffs so that businesses can apply for waivers to import products that cannot be obtained from domestic sources. The president may sign the official tariff order as soon as Thursday, but details of additional exclusions may not be ready for 30 days, according to one former U.S. trade official.

When the U.S. last imposed tariffs on imported steel in 2002, George W. Bush's administration had a waiver process in place six months before the tariffs took effect.

"In the short term, it's going to be chaotic," said William Reinsch, a former Commerce Department official who is now at the Center for Strategic and International Studies.

The situation left lobbyists and economists alike unsure of the road ahead. By themselves, the tariffs are likely to have little impact on a $20 trillion economy that is already at full employment.

But with the European Union, Canada and China vowing retaliation, there is a danger that a costly global conflict could erupt, said Jim O'Sullivan, chief U.S. economist for High Frequency Economics, who expects the economy to expand by 2.9 percent this year.

"My fairly positive forecast for the economy assumes it does not turn into a major confidence-sapping trade war, with equities plunging etc.," O'Sullivan said via email. "I think the equity market will be an important signaling device here."

For now, the stock market - after plunging by 586 points when the tariffs were announced - appears sanguine. The Dow Jones industrial average fell by less than 1 percent on Wednesday, and remains higher than on March 1, when Trump first disclosed his tariff plan.

The president signaled in a pair of tweets that his attention may soon shift to China. "The U.S. is acting swiftly on Intellectual Property theft. We cannot allow this to happen as it has for many years!" he wrote.

The administration has been considering for weeks various measures intended to punish China for compelling foreign companies to surrender their trade secrets in return for access to the world's second-largest economy.

Among the options under review are tariffs on a range of Chinese products and new restrictions on Chinese investment in the U.S., according to individuals familiar with the discussions.

The most radical steps would attempt to unwind existing Chinese investments, not just limit new ones, but that would raise legal questions, they said.

The debate follows Trump's request in August for Robert Lighthizer, the U.S. trade representative, to examine whether China's intellectual property policies unfairly discriminated against U.S. companies. That probe could continue until August, but a decision is expected by the end of this month, according to a former U.S. trade negotiator.

In striking at China, the administration can expect more uniform backing from corporate leaders, who have grown frustrated with the country's less welcoming stance under President Xi Jinping.

Earlier in the day, the president also tweeted inaccurately about the overall trade balance between the U.S. and China.

"China has been asked to develop a plan for the year of a One Billion Dollar reduction in their massive Trade Deficit with the United States. Our relationship with China has been a very good one, and we look forward to seeing what ideas they come back with. We must act soon!" the president wrote.

In fact, China last year had a $375 billion trade surplus with the U.S. Reducing it by $1 billion would have no appreciable economic consequences.

The overall U.S. trade deficit rose to $56.6 billion in January, the highest monthly figure in more than nine years, the Commerce Department said.

Steel industry leaders are split over whether to exclude Canada. The United Steelworkers union, which has numerous members in Canada, is urging the administration to exclude Canada, arguing that America's northern neighbor trades fairly.

"There's no rational reason to have Canada sanctioned because Canada never broke the rules," said Leo Gerard, international president of the USW. "We want to encourage the administration to go after the cheaters."

But steel executives are more leery of an exemption for Canada, the largest source of imported steel. The country's mills could become an even larger source as companies in America scramble to find new suppliers if the tariff only goes into effect on Asia and Europe.

Trump should do a "fixed tariff across the board," said Dan DiMicco, chairman emeritus of Nucor Corporation, one of America's largest steel producers. "If I had my way, we'd be doing a 50 percent tariff on the really bad actors and 20 percent on the rest."

President George W. Bush exempted Canada and Mexico from his steel tariffs in 2002. At the time, Bush's advisers felt it wouldn't be fair to put the tariff on such close allies and free trading partners. However, economists say there were still negative impacts on the economy, even with the carve-out for Canada and a handful of other nations and products.

"I don't think it's a game changer to get Canada excluded from tariffs," said Doug Holtz-Eakin, who has advised GOP candidates for president. "We did that under Bush. It was still the case that domestic harm to steel-consuming industries outweighed the gains to steel producers."

The Washington Post's Erica Werner, Damian Paletta, Robert Costa and Josh Dawsey contributed to this report.